1. Field of the Invention
This invention relates to an improvement in and relating to various elements of an electrostatic copying apparatus.
2. Description of the Prior Art
As document copying has been diversified in recent years, an electrostatic copying apparatus of the type capable of forming a copied image on both surfaces of a copying paper sheet has been proposed and come into commercial acceptance. This type of electrostatic copying apparatus, as is well known to those skilled in the art, comprises a copying paper conveying passage, a copying paper feeding means for feeding copying paper to the copying paper conveying passage, a copying paper returning passage, and a copying paper re-sending means for receiving the copying paper sheet returned through the copying paper returning passage and re-sending the received copying paper sheet to the copying paper conveying passage, and is adapted to form a copied image on both surfaces of the copying paper sheet in the following manner. The copying paper sheet fed to the paper conveying passage from the paper feeding means is conveyed through the paper conveying passage and during this conveyance, an image is formed on one surface of the paper sheet. The paper sheet having the image formed on its one surface is then selectively introduced into the paper returning passage and sent to the paper re-sending means. The paper sheet thus received in the paper re-sending means is re-sent to the paper conveying passage by the action of the paper re-sending means. While it is again conveyed through the paper conveying passage, an image is formed on the other surface of the copying paper sheet. As a result, the desired images are formed on both surfaces of the copying paper sheet fed from the paper feeding means.
The aforesaid type of electrostatic copying apparatus known in the art has the following inconveniences or defects.
Firstly, the paper returning means usually includes a copying paper receiving stand for receiving copying paper sheets returned through the paper returning passage. To receive the copying paper sheets in the desired stacked state on the paper receiving stand, the copying paper sheets should be conveyed with a relatively weak sending force. On the other hand, in order to send the uppermost sheet of the stack received on the paper receiving stand toward the paper conveying passage, it should be delivered with a relatively strong sending force. In the conventional electrostatic copying apparatus of this type, the paper returning means further includes auxiliary conveying rollers for receiving the copying paper sheets on the paper receiving stand in the desired conditions (these auxiliary conveying rollers act relatively weakly on the copying paper sheet and convey it) and delivery rollers for delivering the copying paper sheet from the paper receiving stand (the delivery rollers act relatively strongly on the copying paper sheet and deliver it), and is constructed such that the auxiliary conveying rollers and the delivery rollers are each selectively held in an operating state in which they act on the copying paper sheet and in a non-operating state in which they are apart from the copying paper sheet. Specifically, it is of such a construction that when the copying paper sheet is received by the receiving stand, the auxiliary conveying rollers are maintained in the operating state and the delivery rollers are maintained in the non-operating state; that when the position of the copying paper sheet in the width direction is set, the auxiliary conveying rollers and the delivery rollers are both kept in the non-operating states; and that when the copying paper sheet is delivered from the receiving stand, the auxiliary conveying rollers are maintained in the non-operating state and the delivery rollers are maintained in the operating state. Thus, in the conventional electrostatic copying apparatus, the auxiliary conveying rollers and the delivery rollers exist independently, and a mechanism is required which selectively maintains the auxiliary conveying rollers and the delivery rollers in the operating states and in the non-operating states. Accordingly, the structure and controlling of the paper re-sending means become complex.
Secondly, in an electrostatic copying apparatus not necessarily limited to the above-described type, a mechanism for preventing the conveying of copying paper sheets in superposed state is well known in order to accurately feed copying paper sheets one by one from the stack. This mechanism is composed of a combination of a conveying roller to be rotated in a predetermined direction and a frictional member to be in pressing contact with the peripheral surface of the conveying roller. This conventional preventing mechanism is of such a structure that the frictional member is always in pressing contact with the peripheral surface of the conveying roller. The frictional member is therefore worn within a relatively short period of time and the aforesaid preventing function of the preventing mechanism will be reduced. As a result, the copying paper sheets cannot be fed accurately one by one.
Thirdly, electrostatic copying apparatuses, not necessarily limited to the above-described type, include a copying paper detecting mechanism for detecting a copying paper sheet conveyed through the paper conveying passage, etc. in order to detect paper jamming in the paper conveying passage or elsewhere. This type of paper detecting mechanism known in the art generally includes a detecting means comprising a combination of a microswitch or light emitting element and a light-receiving element and is constructed such that the detecting means directly detects passage of the copying paper sheet. For example, to detect paper jamming in the paper returning passage of the aforesaid type of electrostatic copying apparatus, it is necessary to annex a copying paper detecting mechanism to the paper receiving stand in relation to the paper returning means and to detect the arrival of the copying paper sheet. However, the conventional paper detecting mechanism can detect the first sheet of the paper stack, but cannot detect the second subsequent sheets because the copying paper sheets are stacked on the receiving stand.